Sugars (in solution - not crystals) heat up fastest of all in a microwave oven. Probably the effect of all those hydroxyl groups interacting with the water. Put a tablespoon of syrup or jam in a microwave, blast it for 10 seconds and it is already boiling, whereas a similar quantity of water is still lukewarm.

Because of this, warnings are placed on (British) Christmas puddings. They are inclined to catch fire if cooked for too long in the microwave, and people just don't anticipate that they only require about a third of the time of other food items of a similar size/weight to heat up. They have a very high dried fruit content whose sugars heat up very quickly, and burn well!

Because frozen water and solid sugar don't accept microwaves very easily, there is a party trick whereby you enclose a bit of jam inside some ice cream and give it a short blast in the microwave. The jam heats up considerably and the ice cream is almost unaffected, creating hot jam inside cold ice cream.

I was taught a super-delicious and quick chocolate sauce for icecream by my mother which takes advantage of this effect. Take roughly a tablespoon each of butter, syrup (golden treacle) and cocoa powder (a little cinnamon or vanilla essence can be added). Blast it for 10 seconds, and the syrup is much hotter than the butter, so you then have to stir it to melt the butter into the syrup and mix in the cocoa powder, rubbing out any lumps. Another 10 or 20 seconds in the microwave and it is done. Pour it over your ice cream, and it solidifies in wonderfully toffee-like way.